Since their discovery in the early 1990s, a great deal has been learned about the composition and properties of carbon nanotube materials. This research has demonstrated that carbon nanotubes exhibit extraordinary mechanical, electronic and chemical properties, which has stimulated substantial interest in developing applied technologies exploiting these properties. Accordingly, substantial research is directed at developing techniques for organizing, arranging and incorporating carbon nanotube materials into useful functional devices.
Carbon nanotubes are allotropes of carbon comprising one or more cylindrically configured graphene sheets and are classified on the basis of structure as either single walled carbon nanotubes (SWNTs) or multiwalled carbon nanotubes (MWNTs). Typically having small diameters (≈1-30 nanometers) and large lengths (up to several microns), SWNTs and MWNTs commonly exhibit length to diameter ratios of ≈102 to about 107). Carbon nanotubes exhibit either metallic or semiconductor electrical behavior, and the energy band structure of nanotube materials varies considerably depending on their precise molecular structure and diameter. Doped nanotubes having intercalants, such as potassium, have been prepared and the central cavities of nanotubes have been filled with a variety of materials, including crystalline oxide particles, metals, gases and biological materials.
Single walled carbon nanotubes (SWNTs), in particular, are identified as candidates for functional materials in a new generation of high performance passive and active nanotube based electronic devices. SWNTs are made up of a single, contiguous graphene sheet joined with itself to form a hollow, seamless tube, in some cases with capped ends similar in structure to smaller fullerenes. SWNTs typically have very small diameters (≈1 nanometer) and are often present in curled, looped and bundled configurations. SWNTs are chemically versatile materials capable of functionalization of their exterior surfaces and encapsulation of materials within their hollow cores, such as gases or molten materials.
A number of unique properties of SWNTs make these materials particularly attractive for a variety of emerging applied technologies, including sensors, light emissive systems, flexible electronics and novel composite materials. First, SWNTs are believed to have remarkable mechanical properties, such as tensile strengths at least 50 times that of steel. Second, the electron transport behavior in SWNTs is predicted to be essentially that of a quantum wire, and the electrical properties of SWNTs have been observed to vary upon charge transfer doping and intercalation, opening up an avenue for potentially tuning the electrical properties of nanotube materials. Finally, SWNTs have also been demonstrated to have very high intrinsic field affect mobilities (e.g., about 10,000 cm2V−1s−1) making them interesting for possible applications in nanoelectronics.
The astonishing electronic and mechanical properties of SWNTs, together with the ability to deposit nanotubes onto plastics and other unusual device substrates, make them well-suited for use in large-scale distributed electronics for steerable antenna arrays, flexible displays, and other systems. Recent work indicates that random networks of SWNTs can form effective semiconductor layers for thin-film transistor- (TFT-) type devices. Device geometries accessed using random networks of SWNTs are particularly promising for enabling low cost, high-performance devices and device arrays for the field of large area electronics. First, random SWNT networks can be effectively assembled at relatively low costs using a range of solution deposition-based fabrication techniques, including solution casting, ink jet printing and screen printing. Second, device geometries employing random SWNT networks for semiconductor channels in electronic devices are also compatible with low temperature assembly on a range of substrates, including flexible polymer substrates desirable for applications of flexible electronics.
Despite substantial progress in developing a SWNT-based electronic device platform using random SWNT networks, several factors impede commercialization of these systems. First, the device mobilities that have been achieved with these networks are far below the intrinsic tube mobilities inferred from measurements of transistors that incorporate an individual tube (or small number of tubes) spanning the gap between the source and drain electrodes. The resistance at the many tube-tube contacts that are inherent in the networks may limit charge transport. Second, films comprising random SWNT networks are typically a mixture of metallic tubes and semiconducting tubes. The presence of metallic tubes in the network often results in a significant extent of purely metallic conductive pathways between the source/drain (S/D) electrodes of SWNT-based thin-film transistor (TFT) devices. Such metallic conductive pathways decrease the device on/off ratio attainable and generally increase the static power consumption, thereby preventing their applications for important classes of electronics systems.
U.S. Pat. No. 7,226,818, issued on Jun. 5, 2007, discloses field-effect transistors based on random SWNT networks enriched in semiconducting nanotubes relative to metallic nanotubes. Enrichment of the semiconducting nanotube component is reported as providing enhanced on/off ratios for thin film transistors based on random SWNT networks. Techniques for enriching the semiconducting nanotube component described in this reference include solution fractionation techniques and selective chemical removal of metallic nanotubes. The authors also report that the channel length can be adjusted to assure that no individual tube spans its length, thus precluding a metallic tube from directly short-circuiting the thin film transistor.
U.S. Pat. No. 6,918,284, issued on Jul. 19, 2005, discloses electronic devices having a semiconductor component comprising an interconnected network or array of carbon nanotubes. The authors exemplify very dilute networks such that at least 75% or substantially all the carbon nanotubes are at least partially in contact with the substrate. The authors report that applying a large source-drain bias while gating off any semiconductive nanotubes can be used to selectively burn metallic nanotubes.
The reference “p-Chanel, n-Channel Thin Film Transistors and p-n Diodes Based on Single Wall Carbon Nanotube Networks”, Nanoletters, Vol. 4, No. 10 (2004) 2031-2035, by Zhou et al., discloses single wall carbon nanotube networks providing semiconductor channels for thin film transistors. The disclosed device geometry is designed to electrically isolate adjacent devices, and the strip widths are wider than the channel lengths.
It will be appreciated from the foregoing that there is currently a need in the art for improved device geometries, components, and fabrication methods to enable passive and active carbon nanotube electronic devices based on random SWNT networks.